This illustrates cutting of the long circumference-type slot, and you can see the tool is just breaking through one side. The part was mounted eccentrically (like a cam) in the 4 jaw chuck. I'll keep edging the tool in deeper until the slot is long enough. In this case, somewhat more than ½ the circumference. I have no idea what the proper amount should be, I just go on what looks correct. I did pre-fit the adapter to the camera so I knew where I wanted the slot to be oriented. The aluminum was wrapped in couple of layers of masking tape to keep its surface pretty. "Neatness counts", ….as our long-suffering teachers used to say.